Wafer cleaning in modern ULSI semiconductor processing presents numerous engineering dilemmas. At various stages during integrated circuit (IC) fabrication, photoresist, residues, and/or particles must be stripped and/or cleaned from wafers. In the evolution of wafer cleaning, the industry previously employed strong acids, bases, solvents, and oxidizing agents in wet chemical procedures. The wet chemical cleaning agents included sulfuric acid, hydrochloric acid, hydrofluoric acid, ammonium hydroxide, hydrogen peroxide, ozone, N-methylpyrrolidone (NMP), and hydroxylamine (HA). Plasma ashing processes have replaced some wet chemical processes due to lower cost, reduced chemical use, and fewer disposal issues. Under the right physical and chemical conditions, plasmas gently and selectively strip and clean wafers. However, for integrated circuits employing certain new materials such as low-dielectric constant (low-k) insulators, traditional plasma ashing is proving too harsh. The new generation of dielectric materials comprises exotic materials such as porous polymers, which cannot withstand the aggressiveness inherent in plasma ashing. Hence a new generation of cleaning processes is being developed.
Among the methods under development are high-pressure processes that employ xe2x80x9cdensifiedxe2x80x9d process solutions. Densified fluids are supercritical or near critical solvents such as supercritical carbon dioxide. The materials are often good solvents for contaminants and residues resulting from semiconductor fabrication.
Although supercritical fluids are generating interest as potential wafer cleaning agents, they present their own unique set of engineering challenges. In particular, the cleaning fluid should strongly but selectively solvate or otherwise strip and clean photoresist, residue, contaminant, or whatever else needs to be removed from the wafer. Also, the solvent should not adsorb to the dielectric material, a problem with traditional wet cleans since it causes the k to shift upward. To this end, some researchers have proposed certain additives to increase the solvating power or otherwise improve the cleaning properties of the densified process fluid.
Some researchers have suggested the addition of a surfactant to supercritical carbon dioxide. See, e.g., U.S. Pat. Nos. 5,944,996 and 5,783,082. Other researchers have suggested the use of oxidizing agents such as oxygen. See JP Patent Document No. S64-45131. Still other researchers have proposed specific additive combinations for removing specific contaminants. See, e.g., U.S. Pat. Nos. 5,868,862, 5,868,856, 6,024,801, and 5,866,005.
While these approaches provide steps in the right direction, they are merely preliminary steps. So far the research has failed to provide a systematic analysis of densified solutions and their usefulness in wafer cleaning processes. Therefore, certain advances remain to be discovered in densified solution cleaning technology.
This invention provides certain formulations of supercritical solutions and their use in wafer cleaning processes. The solutions described in this document may be used to clean many different types of waste from IC fabrication procedures. The waste includes organic and inorganic materials. These materials include photoresists, residues and particles. They may be polymers, metals, organometallics, metal oxides, semiconductors, semiconductor oxides, oxides of dopants, etc.
Particularly preferred embodiments for cleaning photoresist and etch residues from low-k materials are described herein. Such supercritical solutions of the invention may be categorized by their chemistry, for example, basic, acidic, oxidative, and fluorinating chemistries are used. In some embodiments, combinations of these chemistries are used. Supercritical solutions of the invention preferably include supercritical carbon dioxide and at least one reagent dissolved therein to facilitate removal of waste material from wafers, particularly for removing photoresist and post-etch residues from low-k materials. For these embodiments, the reagent preferably includes at least one of a basic ammonium compound (e.g., an ammonium hydroxide, ammonium carbonate, or ammonium bicarbonate), an organic acid, a peroxide source, a fluoride ion source, and combinations of such reagents. As well, the solution may include one or more co-solvents, chelating agents, surfactants, and anti-corrosion agents as well.
One aspect of the invention pertains to methods of removing waste material from a semiconductor wafer. These methods may be characterized as follows: (a) receiving the semiconductor wafer in a chamber; and (b) providing a supercritical solution to the chamber to thereby contact at least one surface of the wafer and remove at least a portion of the waste material from the semiconductor wafer. In these methods, the supercritical solution includes a supercritical solvent with a reagent or reagents dissolved therein to facilitate removal of the material. This reagent may be an ammonium bicarbonate or carbonate, and combinations thereof. Preferably the supercritical solvent is supercritical carbon dioxide, but the invention is not limited in this way.
Also, preferably the supercritical solution may further include a co-solvent that increases the solubility of the reagent (or waste material) in the supercritical solvent. Preferably the co-solvent is selected from the group consisting of alcohols, ethers, alkyl halides, alkanes (straight, branched, or cyclic), alkenes (straight, branched, or cyclic), aromatic compounds, highly fluorinated hydrocarbons (e.g., C6F14), siloxanes, nitriles, amides, and combinations thereof. Depending upon which chemistry i.e. reagents or combinations of reagents are used in a particular supercritical solution, a particular co-solvent or combination of co-solvents may be preferred.
Preferably providing the supercritical solution to the chamber to thereby contact at least one surface of the wafer and remove at least a portion of the waste material from the semiconductor wafer includes adding the reagent, as a solution in the co-solvent, to the supercritical solvent in the presence of the semiconductor wafer.
Ammonium bicarbonates and carbonates of the invention are preferably tetraalkyl ammonium compounds, but the invention is not limited in this way. A particularly preferred ammonium bicarbonate is tetramethyl ammonium bicarbonate, which is particularly soluble in carbon dioxide. Particularly preferred methods using these reagents and combinations thereof, are de scribed below more fully in the detailed description.
In some preferred embodiments, methods of the invention further include rinsing the semiconductor wafer with at least one of deionized water, an organic solvent, the supercritical solvent, and mixtures thereof after providing the supercritical solution to the chamber to thereby contact at least one surface of the wafer and remove at least a portion of the waste material from the semiconductor wafer.
In many preferred embodiments, the supercritical solution also includes a chelating agent that complexes metal ions contained in the waste material to be removed from the wafer. Generally, any conventional chelating agent used to chelate metal ions in inorganic chemistry may be used. Specific examples of chelating agents suitable for use in this invention include xcex2-diketones, ethers (e.g., crown ethers), phenols (including catechols and sterically hindered phenols such as gallic acid), EDTA (ethylene diamine tetraacetic acid), etc. The chosen chelating agent should not react with the supercritical solvent, the co-solvent, or other reagents used in the supercritical solution.
To protect against corrosion of exposed metal surfaces on the wafer (during wafer transport or storage for example), the supercritical solution may include an anti-corrosion agent. Many chelating agents will also serve as anti-corrosion agents. However, the agent must strongly adhere to the metal""s surface. Not all chelating agents meet this requirement. Often anti-corrosion agents will possess bulky side-groups and provide steric hindrance. Examples of suitable anti-corrosion agents include ortho- and meta-dihydroxybenzenes such as catechol, gallic acid, oxalic acid, thiophenes, thiols, glymes, etc.
Finally, the supercritical solution may include a surfactant that assists in removal of particles from the at least one surface of the wafer. Examples of suitable surfactants for this purpose include amphiphilic fluoropolymers and siloxanes, poly carbonates and carbonate copolymers.
The detailed description below will further discuss the benefits and features of this invention.